Maybe I'm misunderstood, but any of the cast iron parts, using muriatic acid will clean them out like new, really kinda impressive. Doesn't bother the machined surfaces, and when it stops bubbling, the part is likely done. I've heard of block cleaners if you feel the block, pump, intake, or heads has some gunk in it, but I've not used any before. I have, however, had numerous marine engines apart and found water jackets packed with sand. Seems to get in there and then gets baked almost like a ceramic. As for my active boats, I flush them after every use to clean out.